1. Field of the Invention
This invention relates generally to an extreme ultraviolet light source, and more particularly, to a laser-plasma, extreme ultraviolet light source for a photolithography system that employs a liquid spray as the target material for generating the laser plasma.
2. Discussion of the Related Art
Microelectronic integrated circuits are typically patterned on a substrate by a photolithography process, well known to those skilled in the art, where the circuit elements are defined by a light beam propagating through a mask. As the state of the art of the photolithography process and integrated circuit architecture becomes more developed, the circuit elements become smaller and more closely spaced together. As the circuit elements become smaller, it is necessary to employ photolithography light sources that generate light beams having shorter wavelengths and higher frequencies. In other words, the resolution of the photolithography process increases as the wavelength of the light source decreases to allow smaller integrated circuit elements to be defined. The current state of the art for photolithography light sources generate light in the extreme ultraviolet (EUV) or soft x-ray wavelengths (13.4 nm).
Different devices are known in the art to generate EUV radiation. One of the most popular EUV light sources is a laser-plasma, gas condensation source that uses a gas, typically Xenon, as a laser plasma target material. Other gases, such as Krypton, and combinations of gases, are known for the laser target material. The gas is forced through a nozzle, and as the gas expands, it condenses and forms a cloud or jet of extremely small particles known in the art as clusters. The condensation of cluster jet is illuminated by a high-power laser beam, typically from a Nd:YAG laser, that heats the clusters to produce a high temperature plasma which radiates the EUV radiation. U.S. Pat. No. 5,577,092 issued to Kublak discloses an EUV radiation source of this type.
FIG. 1 is a plan view of an EUV radiation source 10 including a nozzle 12 and a laser beam source 14. FIG. 2 is a close-up view of the nozzle 12. A gas 16 flows through a neck portion 18 of the nozzle 12 from a gas source (not shown), and is accelerated through a narrowed throat portion 20 of the nozzle 12. The accelerated gas 16 then propagates through a flared portion 24 of the nozzle 12 where it expands and cools, and is expelled from the nozzle 12. As the gas cools and condenses, it turns into a jet spray 26 of clusters 28.
A laser beam 30 from the source 14 is focused by focusing optics 32 on the clusters 28. The heat from laser beam 30 generates a plasma 34 that radiates EUV radiation 36. The nozzle 12 is designed so that it will stand up to the heat and rigors of the plasma generation process. The EUV radiation 36 is collected by collector optics 38 and is directed to the circuit (not shown) being patterned. The collector optics 38 can have any suitable shape for the purposes of collecting the radiation 36, such as a parabolic shape. In this design, the laser beam 30 propagates through an opening 40 in the collector optics 38.
The laser-plasma EUV light source discussed above suffers from a number of drawbacks. Particularly, it is difficult to produce a sufficiently large droplet spray or large enough droplets of liquid to achieve the desirable efficiency of conversion of the laser radiation to the EUV radiation. Because the clusters 28 have too small a diameter, and thus not enough mass, the laser beam 30 causes some of the clusters 28 to break-up before they are heated to a sufficient enough temperature to generate the EUV radiation 36. Typical diameters of the droplets generated by a gas condensation EUV source are less than 0.01 microns and it is exceedingly difficult to produce clusters that are significantly larger than 0.1 microns. However, particle sizes of about one micron in diameter would be more desirable for generating the EUV radiation. Additionally, the large degree of expansion required to maximize the condensation process produces a diffuse cloud or jet of clusters, and is inconsistent with the optical requirement of a small plasma size.
What is needed is a laser-plasma EUV radiation source that is able to generate larger droplets of liquid to enhance the EUV radiation generation. It is therefore an object of the present invention to provide such an EUV radiation source.